Primary & Foreign Keys
PRIMARY KEY, HAR, ROW, ko, UNIQUELY, IDENTIFY, karta hai — EK, TABLE, mein, SIRF, EK, primary, key, ho, sakti, hai (SINGLE, ya, MULTIPLE, columns, se, ban, sakti, hai). AUTO_INCREMENT, se, AUTOMATICALLY, INCREMENTING, values, generate, hoti, HAIN.
FOREIGN KEY, EK, TABLE, ke, COLUMN, ko, DUSRI, TABLE, ke, primary, key, se, LINK, karta hai — REFERENTIAL, INTEGRITY, ENSURE, karta hai (jaise, EK, INVALID, user_id, waala, order, INSERT, NAHI, ho, sakta).
CREATE TABLE orders (
id INT PRIMARY KEY AUTO_INCREMENT,
user_id INT,
FOREIGN KEY (user_id) REFERENCES users(id)
ON DELETE CASCADE
);
-- ON DELETE CASCADE: user delete hone par, uske orders, bhi, delete, ho, jaayenge- Primary key = row, ko, uniquely, identify, karta hai
- Foreign key = dusri, table, ke, row, ko, reference, karta hai
- Referential integrity, ensure, karta hai — invalid, references, allow, nahi, hote
Primary, key, MULTIPLE, columns, se, ban, sakti, hai — jaise, EK, junction, table, mein, (student_id, course_id) DONO, milkar, primary, key, banaте, hain, HAR, combination, ko, UNIQUE, ensure, karte, hue.
CREATE TABLE enrollments (
student_id INT,
course_id INT,
PRIMARY KEY (student_id, course_id)
);ON DELETE/ON UPDATE, ke, saath, RESTRICT (default), CASCADE, SET NULL, ya, NO ACTION, actions, SPECIFY, kiye, ja, sakte, hain — decide, karte, HAIN, ki, PARENT, row, change/delete, hone, PAR, CHILD, rows, ka, KYA, hoga.